It is well known to use an electric motor to cause a shifting of a servovalve's spool. This is usually accomplished through a mechanical link that converts the rotary motion of the motor's output shaft into a linearly-directed force that acts on the valve's spool. One example of such a mechanical link is an offset tip of the motor's shaft engaging a groove/aperture in the spool. In this manner, rotation of the shaft causes the tip to move in an arc, thereby applying a force on the spool that is at least partially directed along the spool's longitudinal axis.
One problem with a mechanical link that employs an offset tip of the motor's shaft is that there can be significant backlash in the connection between the tip and the valve. This is usually due to the tip having a single linear contact with the shaped groove/aperture in the valve's spool. When the rotation of the motor's shaft is reversed, any play whatsoever between the tip and the sides of the spool's groove/aperture will allow the tip to move without a concomitant movement of the spool.
One method used in the prior art to overcome the above-noted problem is to fully retain the shaft's tip within a bushing located in the spool's receiver. This is taught by Spurbeck in U.S. Pat. No. 4,573,494. However, this is only a temporary solution since backlash will arise as soon as the bushing wears. In addition, the extra parts increase the valve's cost and maintenance requirements.
A second problem with prior art direct-drive valves is that it is both necessary and extremely difficult to precisely limit the amount of rotational movement of the drive motor's shaft. When a valve's spool is shifted due to a rotational movement of a drive motor's shaft, the amount of rotation determines the length of the valve's stroke (translation of the spool). If the motor's shaft rotates to a lesser or greater extent than is required, the spool may not shift a full stroke, or will shift too far, or may even shift a full stroke and then reverse direction and partially retrace its path. Therefore, precisely limiting the motor's rotation is absolutely critical to proper valve function.
There have been a number of methods employed in the prior art to limit the amount of rotation of the motor's output shaft. Most commonly, the motor includes internal stops that stop the rotor's movement. However, the stops can break or wear, resulting in improper rotation of the motor's shaft.
Another method for limiting the rotation of the motor's output shaft is taught by Hair et al in U.S. Pat. No. 5,040,568. The patent teaches the use of a shaped cam plate that is attached to the tip portion of the motor's shaft. When the motor is attached to the valve body, the plate is received within a specially-shaped cavity in the valve body. As the tip rotates, it causes the plate to shift within the cavity. The tip movement, and hence the motor's rotation, is stopped when a side of the plate abuts a sidewall of the cavity. While this is an effective method for limiting the rotation of the motor, it requires the use of a cam plate that must be precisely machined and secured to the shaft in a slip-free manner. Furthermore, the body of the valve must include a precisely machined cavity for receiving the plate. Once the plate is within the cavity, the cavity must remain free of corrosion and dirt, since any foreign material on the contact surfaces would adversely affect proper operation of the valve. In addition, any wear of the cam plate, of the connection between the plate and shaft, or of the cavity's sidewalls will result in inaccurate movement of the valve's spool. Additionally, the added parts and precise machining increase the valve's cost and its maintenance requirements.